Copernicus mentions as his allies only the ancient philosophers Philolaus and Niketas.

The next period of development of astronomy is associated with the activities of Islamic scholars – al-Battani, al-Biruni, Abu al-Hassan ibn Eunis, Nasir hell-Din at-Tusi, Ulugbek and many others.

Formation of theoretical astronomy. Middle ages

In the Middle Ages, astronomers were engaged only in observing the visible motions of the planets and coordinating these observations with the accepted geocentric system of Ptolemy.

Interesting cosmological ideas can be found in the works of Origen of Alexandria, a prominent apologist of early Christianity, a student of Philo of Alexandria. Origen urged to take the Book of Genesis not literally, but as a symbolic text. The universe, according to Origen, contains many worlds, including inhabited. Moreover, he allowed the existence of many universes with their stellar spheres. Every universe is finite in time and space, but the very process of their origin and death is infinite:

As for me, I will say that God did not begin his work when our visible world was created; and just as after the end of the existence of the latter another world arises, so before the beginning of the Universe there existed another Universe. Thus, it should be assumed that not only do many worlds exist at the same time, but before the beginning of our universe there were many universes, and after its end there will be other worlds.

I must say, it sounds quite modern.

In the XI-XII centuries the main scientific works of the Greeks and their Arabic-speaking students were translated into Latin. The founder of scholasticism, Albert the Great, and his disciple Thomas Aquinas in the thirteenth century dissected the teachings of Aristotle, making it acceptable to the Catholic tradition. From that moment on, Aristotle-Ptolemy’s world system actually merges with Catholic dogmatism. The experimental search for truth was replaced by a method more familiar to theology – the search for appropriate quotations in canonized works and their spatial commentary.

In the XIII century in Toledo under the patronage of King Alfonso X the Wise of Castile opened the first observatory in Europe. It employed Christians, Jews and Muslims; their astronomical tables were published in 1252 (elevated to the king on his accession to the throne). „Alphonse tables“ were of good accuracy and were used for more than two centuries.

In the XV century, the German philosopher, Cardinal Nicholas of Cusa, significantly ahead of his time, expressed the view that the universe is infinite, and it has no center at all – neither the Earth, nor the Sun, nor anything else occupy a special position. All celestial bodies consist of the same matter as the Earth, and, quite possibly, lived. A century before Galileo, he argued that all luminaries, including the Earth, move in space, and every observer on it has the right to be considered motionless.

In the 15th century, the works of Georg Purbach, as well as his student and friend Johann Müller (Regiomontana), played an important role in the development of observational astronomy. By the way, they became the first scientists in Europe who did not have a spiritual rank. After a series of observations, they were convinced that all the astronomical tables that were, including Alfonsino, were outdated: the position of Mars was given with an error of 2 °, and the lunar eclipse was delayed by an hour! To increase the accuracy of calculations Regiomontan made a new table of sines (after 1 ‚) and a table of tangents. The newly published book printing contributed to the fact that Purbach’s revised textbook and Regiomontan’s Ephemeris had been the main astronomical guide for Europeans for decades. Regiomontan’s tables were much more accurate than before and served well up to Copernicus. They were used by Columbus and Amerigo Vispucci. Later, the tables were even used for some time for calculations on the heliocentric model.

Regiomontan also proposed a method for determining longitude by the difference between tabular and local time, corresponding to a given position of the Moon. He noted the discrepancy between the Julian calendar and the solar year by almost 10 days, which forced the church to think about calendar reform. Such a reform was discussed at the Lateran Council (Rome, 1512-1517) and was implemented in 1582.

The copernican revolution

By the sixteenth century, it was clear that Ptolemy’s system was inadequate and led to unacceptably large calculation errors. To increase the accuracy of calculations of the positions of the planets, some astronomers have proposed the introduction of additional epicycles, but they did not save the position. Nicolaus Copernicus was the first to propose an alternative that worked in detail, based on a completely different model of the world.

The main work of Copernicus – „De Revolutionibus Orbium Caelestium“ (On the rotation of the celestial spheres) – was largely completed in 1530, but only before his death Copernicus decided to publish it. However, in 1503-1512, Copernicus distributed to friends a handwritten synopsis of his theory („a small commentary on hypotheses relating to celestial motions“), and his student Retic published a clear statement of the heliocentric system in 1539. Apparently, rumors of a new theory spread widely in the 1520s.

In structure, the main work of Copernicus almost repeats the „Almagest“ in a somewhat abbreviated form (6 books instead of 13). The first book also gives axioms, but instead of the position of the Earth’s immobility, another axiom is placed – the Earth and other planets revolve around the axis and around the Sun. This concept is argued in detail, and the „opinion of the ancients“ is more or less convincingly refuted. Copernicus mentions as his allies only the ancient philosophers Philolaus and Niketas.

From a heliocentric position, Copernicus easily explains the rotational motion of the planets. The following is the same material as Ptolemy, only slightly refined: spherical trigonometry, stellar catalog, the theory of motion of the Sun and Moon, an estimate of their size and distance to them, the theory of precession and eclipses.

In Book III, devoted to the annual motion of the Earth, Copernicus makes an epoch-making discovery: he explains the „precedence of the equinoxes“ by a shift in the direction of the earth’s axis. In books V and VI, devoted to the motion of the planets, thanks to the heliocentric approach, it became possible to estimate the average distances of the planets from the Sun, and Copernicus provides these data, quite close to modern ones.

The system of the world of Copernicus, from the modern point of view, is not yet radical enough. All orbits are circular, the movement on them is uniform, so the epicycles had to be preserved – however, instead of 80 they became 34. The mechanism of rotation of the planets is preserved the same – the rotation of the spheres to which the planets are attached. But then the axis of the Earth during the annual rotation must rotate, describing the cone; to explain the change of seasons, Copernicus had to introduce a third (reverse) rotation of the Earth around an axis perpendicular to the ecliptic, which he also used to explain precession. Copernicus placed the sphere of fixed stars on the edge of the world.

Strictly speaking, Copernicus‘ model was not even heliocentric, because he did not place the Sun in the center of the planetary spheres.

Copernicus naturally ruled out Ptolemy’s shift in the center of its orbit, and this was a step backwards – initially more accurate than Ptolemy’s, Copernicus‘ tables soon diverged significantly from observations, much to the bewilderment and chill of its enthusiastic supporters. Yet in general, Copernicus‘ model of the world was a colossal step forward and a devastating blow to archaic authorities.

The Catholic Church was at first sympathetic to the revival of the Pythagoreans, and some of its pillars even patronized Copernicus. Father Clement VII, concerned about the clarification of the calendar, instructed Cardinal Wigmanstadt to give a lecture to the high clergy on a new theory, which was listened to carefully. There were, however, among Catholics and ardent opponents of heliocentrism. However, as early as the 1560s, lectures on the Copernican system began at several universities in Switzerland and Italy. The mathematical basis of the Copernican model was somewhat simpler than that of Ptolemy, and this was immediately used for practical purposes: refined astronomical („Prussian“) tables (1551, E. Reingold) were published.

Of the other events of the turbulent XVI century, we note that on October 5, 1582, a long-planned calendar reform was carried out (October 5 was the 15th). The new calendar was named Gregorian after Pope Gregory XIII, but the real author of the project was the Italian astronomer and physician Luigi Lillio.

The invention of the telescope. Galileo

The great Italian scientist Galileo Galilei accepted the Copernican system with enthusiasm, and immediately rejected the fictitious „third movement“, showing by experience that the axis of a moving spinning top retains its direction by itself. He decided to use a telescope to prove Copernicus right.

Polished glass lenses were known to the Babylonians; the oldest of the lenses found during excavations dates back to the VII century BC. In 1608 a telescope was invented in Holland; learning of this in the summer of 1609, Galileo independently built a much improved version of it, creating the world’s first refracting telescope. The magnification of the telescope was initially threefold, later Galileo brought it to 32 times.

Galileo expressed the sensational results of his research in a series of articles „Star Herald“ (1610), causing a real flurry of optical observations of the sky among scientists. It turned out that the Milky Way consists of clusters of individual stars, that the Moon has mountains (up to 7 km high, which is close to the truth) and depressions, the Sun has spots, and Jupiter – satellites (the term „satellite „was later introduced by Kepler). Of particular importance was the discovery that Venus has phases; in Ptolemy’s system Venus as the „lower“ planet was always closer to the Earth than the Sun, and „fullness“ was impossible.

Galileo noted that the diameter of stars, unlike planets, does not increase in the telescope, and some nebulae, even in an enlarged form, do not break up into stars; this is a clear sign that the distances to the stars are enormous even compared to the distances in the solar system.

Galileo found in Saturn performances, which he took for two satellites. Then the speeches disappeared (the ring turned), Galileo considered his supervision an illusion and did not return to this topic; Saturn’s ring was discovered in 1656 by Christian Huygens.

Galileo did not accept Kepler’s ellipses, continuing to believe in the circular orbits of the planets. The reason for this may have been Kepler’s excessive fascination with mystical numerology and „world harmony“.